A circuit for tuning superconductor-insulator-superconductor mixers using distributed tunnel junctions

A circuit that produces broadband impedance matching with low-current-density junctions at submillimeter wavelengths for tuning superconductor-insulator-superconductor (SIS) mixers is described; it consists of half-wavelength distributed SIS junctions connected alternately by,half-wavelength microstrip lines. It is made of Nb and was designed for two practical source impedances using numerical calculation based on the Mattis-Bardeen theory of the anomalous skin effect at 660 GHz, assuming SIS junctions with a current density of 5 kA/cm(2). An SIS mixer tuned using this circuit had a large fractional bandwidth of more than 18%, below -10 dB matching, even though the assumed omegaC(J)R(N) product was about 16. Investigation of the mixer performance using Tucker's quantum theory of mixing showed that it has excellent broadband and low-noise characteristics compared with conventional mixers consisting of half- or full-wavelength distributed elements.